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Neutral Current Analysis in MINOS

Neutral Current Analysis in MINOS. Alexandre Sousa University of Oxford Harvard University* DOE Review of the Harvard HEP Group August 15, 2008. *After December 01, 2008. Neutrino Interactions in MINOS.

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Neutral Current Analysis in MINOS

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  1. Neutral Current Analysis in MINOS Alexandre Sousa University of Oxford Harvard University* DOE Review of the Harvard HEP Group August 15, 2008 *After December 01, 2008

  2. Neutrino Interactions in MINOS • The MINOS detectors observe both neutral current (NC) and charged current (CC) interactions • Reconstructed events are composed of tracks and showers • MINOS is not optimized to measure the short showering NC events, but they are interesting! • NC events allow us to look for sterile neutrinos Hadronic Interaction Length = 7 planes Range of 2 GeV muon = 50 planes EM Radiation Length = 0.7 steel planes Alex Sousa

  3. Sterile Neutrinos • Measurements of the Z0 decay width at LEP exclude more than 3 light active neutrinos • A 4th neutrino cannot couple to Z0 • It does not participate in weak interactions – sterile neutrino • Results from the short baseline LSND experiment suggested the existence of a fourth neutrino with large mass splitting • Results from MiniBooNE and other experiments strongly disfavor sterile neutrinosas an explanation for LSND • Searches on long baseline experiments are relevant as sterile neutrinos would • Indicate presence of one or more additional mass eigenstates • be possible dark matter candidates • Sterile neutrinos => new physics! Phys. Rev. Lett. 98, 231801 (2007) Alex Sousa

  4. No νs With νs mixing Toy Simulation Looking for Sterile Neutrinos in MINOS • Oscillations in MINOS are driven by Δm2Atm • Oscillation is between νμ and ντ • No effect on neutral current interactions • Add a 4th neutrino • Extra mass ν4, extra flavor νs • Oscillations can now occur between νμ and νs • driven by Δm2Atm • driven by a new mass scale • Oscillations into νs reduce number of observed NC interactions as νs do not interact in the detector • Look for NC disappearance at the Far Detector • Sterile neutrino mixing would deplete NC Energy spectrum Alex Sousa

  5. Beam quality and detector quality cuts Beam positioning, magnetic horns energized, detector running within operational parameters Event vertex reconstructed within the fiducial volume of the detectors Fiducial volume is optimized for containment of hadronic showers Cut-based method is used to separate CC and NC events NC Event Selection Calorimeter Spectrometer n NEAR DETECTOR n FAR DETECTOR Alex Sousa

  6. NC/CC Event Separation • NC events are typically shorter than CC events • Expect showers and no tracks or very short tracks reconstructed for NC events • Main background from inelastic (high-y) νμCC events • Event classified as NC-like if: • event length < 60 planes • has no reconstructed track or • has one reconstructed track that does not protrude more than 5 planes beyond the shower Excluded Excluded Excluded Alex Sousa

  7. The FD NC selection uses the same variables as the ND selection, with identical cut values MC oscillated with 2007 CC best fit: Dm2= 2.38 x10-3 eV2, sin2(2q23)=1 Far Detector NC Selection Excluded Excluded Excluded Far Detector Data Osc. Monte Carlo Alex Sousa

  8. NC Selected Events in Far Detector Data Alex Sousa

  9. Extrapolation to the Far Detector • Far detector (FD) energy spectrum without oscillations is not the same as the Near detector (ND) spectrum • Decay angles for neutrinos to reach detector are different for ND and FD  different energy spectrum • The measured ND energy spectrum is used to predict the unoscillated FD energy spectrum. Corrections for energy smearing and detector acceptance are obtained from MC • Ratio of events in a given bin in FD relative to ND is the same for data and MC • F/N Ratio method is used to cancel most systematic uncertainties on flux and cross-sections π+ p Target FD Decay Pipe ND En ~ 0.43Eπ/ (1+gπ2θn2) Alex Sousa

  10. NC Analysis Results - Rate • Compare the NC energy spectrum with the expectation of standard 3-flavor oscillation physics • Fix the oscillation parameter values • sin22Θ23 = 1 • Δm232= 2.43x10-3 eV2 • Δm221 = 7.59x10-5 eV2, Θ12 = 0.61 from KamLAND+SNO • Θ13 = 0 or 0.21 (normal MH, δ=3π/2) from CHOOZ Limit • N.B. CC ne are classified as NC by the analysis • Make comparisons in terms of the R statistic: • For different energy ranges • 0-3 GeV • 3-120 GeV • All events (0-120 GeV) From MINOS 2008 CC measurement Predicted CC background from all flavors Predicted NC interaction signal Alex Sousa

  11. NC Analysis Results - Rate • Plot shows the selected FD NC energy spectrum for Data and oscillated MC predictions • Expect largest NC disappearance for E < 3 GeV if sterile mixing is driven by Δm232 • Depletion of total NC event rate (1-R) < 17% at 90% C.L. for the 0-120 GeV range MINOS Far Detector NC Spectrum Data is consistent with no NC deficit at FD and thus with no sterile neutrino mixing Alex Sousa

  12. NC Analysis Results – fs Fit • Assume one sterile neutrino and that mixing between νμ, νs and ντ occurs at a single Δm2 • Survival and sterile oscillation probabilities are then: • Simultaneous fit to CC and NC energy spectra yields the fraction of νμ that oscillate to νs: Alex Sousa

  13. fs Sensitivities • fs fit sensitivities with and without νeappearance and with and without including systematics for different exposures • Solid curves are at 90% C.L., dashed curves are at 68% C.L. VERY PRELIMINARY Alex Sousa

  14. MINOS has completed an analysis of neutral current neutrino interactions in 2.461020 POT of NuMI beam exposure: From 3-flavor analysis: R = 0.99 ± 0.09 ± 0.07, 0 < E < 120 GeV 1-R < 17% at 90% C.L., 0 < E < 120 GeV From fit to a 3+1, single mass splitting, sterile oscillation model Results consistent with no sterile neutrino mixing Submitted to PRL (hep-ex:0807.2424) PRD paper in preparation, including fitting with more complex oscillation models and 3.201020 POT exposure. Next round of analysis to include improved reconstruction and MC Conclusions and Outlook Alex Sousa

  15. Backup Slides Alex Sousa

  16. Systematic Errors • Relative Normalization: 4% • POT counting, Near/Far reconstruction efficiency, fiducial mass • Relative Hadronic Calibration:3% • Inter-Detector calibration uncertainty • Absolute Hadronic Calibration: 11% • Hadronic Shower Energy Scale(6%), Intranuclear rescattering(10%) • Muon energy scale:2% • Uncertainty in dE/dX in MC • CC Contamination of NC-like sample: 15% • NC contamination of CC-like sample: 25% • Cross-section uncertainties: • mA (qe) and mA (res): 15% • KNO scaling: 33% • Poorly reconstructed events: 10% • Near Detector NC Selection: 8% in 0-1 GeV bin • Far Detector NC Selection: 4% if E < 1 GeV, <1.6% if E > 1 GeV • Beam uncertainty: 1s error band around beam fit results Effect of the most relevant systematic uncertainties on R Alex Sousa

  17. Accumulated Beam Data 2009 NC (PRD) 2008 NC (PRL) Higher energy beam 0.15x1020 POT RUN IIa1.23x1020 POT RUN III1.2x1020 POT RUN IIb0.71x1020 POT RUN I - 1.27x1020 POT Alex Sousa

  18. Neutral Current NC Energy Spectrum • NC selected Data and MC energy spectra for Near Detector • Good agreement between Data and Monte Carlo • Discrepancies much smaller than systematic uncertainties • NC events are selected with 90% efficiency and 60% purity Alex Sousa

  19. Simulated Data Systematic Errors • Systematic errors studied using simulated Far Detector data histograms with oscillation parameters Dm2= 2.38 x10-3 eV2, sin22q23=1 • Left plot displays magnitude of shift in FD simulated data compared to nominal • Ratio plots show shifted/nominal ratio for FD simulated data, overlaid with shifted/nominal MC FD prediction • Displays ability of F/N extrapolation method to reproduce systematic shift • F/N extrapolation method is robust to absolute systematic errors, which shift the energy spectra in both Near and Far detectors • Most relevant systematics are relative, where shifts only applied to one detector Alex Sousa

  20. Simulated Data Simulated Data Systematic Errors Alex Sousa

  21. nm to nsterile in SuperK • High energy n experience matter effects which suppress oscillations to sterile n • Matter effects not seen in up-m or high-energy PC data • Reduction in neutral current interactions also not seen • constrains ns component of nm disappearance oscillations • Pure nm->ns disfavored • ns fraction < 20% at 90% c.l. • Result published only in conference proceedings Alex Sousa

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